Ca(2+)-activated nonselective cation channel in apical membrane of vestibular dark cells

Spatiotemporal expression of TRPM4 in the mouse cochlea

The present study was conducted to elucidate the presence of the transient receptor potential cation channel subfamily M member 4, TRPM4, in the mouse inner ear. TRPM4 immunoreactivity (IR) was found in the cell body of inner hair cells (IHCs) in the organ of Corti in the apical side of marginal cells of the stria vascularis, in the apical portion of the dark cells of the vestibule, and in a subset of the type II neurons in the spiral ganglion. Subsequently, changes in the distribution and expression of TRPM4 in the inner ear during embryonic and postnatal developments were also evaluated. Immunohistochemical localization demonstrated that the emergence of the TRPM4-IR in IHCs occurs shortly before the onset of hearing, whereas that in the marginal cells happens earlier, at the time of birth, coinciding with the onset of endolymph formation. Furthermore, semiquantitative real-time PCR assay showed that expressions of TRPM4 in the organ of Corti and in the stria vascularis increased dramatically at the onset of hearing. Because TRPM4 is a Ca(2+) -activated monovalent-selective cation channel, these findings imply that TRPM4 contributes to potassium ion transport, essential for the signal transduction in IHCs and the formation of endolymph by marginal cells.

Stria vascularis and vestibular dark cells: characterisation of main structures responsible for inner-ear homeostasis, and their pathophysiological relations

The regulation of inner-ear fluid homeostasis, with its parameters volume, concentration, osmolarity and pressure, is the basis for adequate response to stimulation. Many structures are involved in the complex process of inner-ear homeostasis. The stria vascularis and vestibular dark cells are the two main structures responsible for endolymph secretion, and possess many similarities. The characteristics of these structures are the basis for regulation of inner-ear homeostasis, while impaired function is related to various diseases. Their distinct morphology and function are described, and related to current knowledge of associated inner-ear diseases. Further research on the distinct function and regulation of these structures is necessary in order to develop future clinical interventions.

MDM2 promotes p21waf1/cip1 proteasomal turnover independently of ubiquitylation

The CDK inhibitor p21waf1/cip1 is degraded by a ubiquitin-independent proteolytic pathway. Here, we show that MDM2 mediates this degradation process. Overexpression of wild-type or ring finger-deleted, but not nuclear localization signal (NLS)-deleted, MDM2 decreased p21waf1/cip1 levels without ubiquitylating this protein and affecting its mRNA level in p53(-/-) cells. This decrease was reversed by the proteasome inhibitors MG132 and lactacystin, by p19(arf), and by small interfering RNA (siRNA) against MDM2. p21waf1/cip1 bound to MDM2 in vitro and in cells. The p21waf1/cip1-binding-defective mutant of MDM2 was unable to degrade p21waf1/cip1. MDM2 shortened the half-life of both exogenous and endogenous p21waf1/cip1 by 50% and led to the degradation of its lysine-free mutant. Consequently, MDM2 suppressed p21waf1/cip1-induced cell growth arrest of human p53(-/-) and p53(-/-)/Rb(-/-)cells. These results demonstrate that MDM2 directly inhibits p21waf1/cip1 function by reducing p21waf1/cip1 stability in a ubiquitin-independent fashion.

Cellular localization of TWIK-1, a two-pore-domain potassium channel in the rodent inner ear

K(+) channels in the inner ear regulate the secretion and homeostasis of K(+), i.e. the flux of K(+) ions required to ensure good mechanosensory transduction. We studied the expression and cellular localization of TWIK-1 and TWIK-2, two-pore-domain K(+) channels responsible for background K(+) currents. Reverse transcription-polymerase chain reaction showed that TWIK-1 mRNA is present in the vestibular end organs, vestibular ganglion and cochlea. In contrast, the TWIK-2 mRNA was not detected in the inner ear. Immunocytochemical experiments using confocal microscopy showed that TWIK-1 is specifically localized in 'non-sensory' cells of the inner ear, in the dark cells of the vestibule and in the strial marginal cells of the cochlea. All of these cell types secrete and regulate the K(+) endolymph production and homeostasis. The labeling was strictly limited to the apical membranes of these cells. TWIK-1 was also detected in the cytoplasm of the large neurons of vestibular ganglion and their fibers. The finding that TWIK-1 is specifically distributed in certain areas of the inner ear suggests that this type of K(+) channel plays a role in the regulation of K(+) homeostasis in dark cells and in strial marginal cells. This role has yet to be identified.

KCNQ1/KCNE1 potassium channels in mammalian vestibular dark cells

The high [K(+)] in the inner ear endolymph is essential for mechanosensory transduction in hearing and balance. Several ion channels, including a slowly activating, voltage-dependent, outwardly conducting K(+) channel composed of the KCNQ1 (KvLQT1) and KCNE1 (IsK/minK) subunits, are expressed at the apical surface of vestibular dark cells. We investigated the underlying molecular mechanisms of this conductance using in situ hybridization, RT-PCR, and immunocytochemistry and by tracking the ultrastructural changes of vestibular structures in kcne1(-/-) mice. In the wild type mice, the KCNE1 and KCNQ1 proteins are expressed specifically at the apical membrane of dark cells, as early as gestational day (GD) 17 for KCNE1 while KCNQ1 mRNAs can be detected at GD 18. This is the first demonstration that the two protein components of this potassium channel co-localize in a polarized fashion at the cellular level. Although the vestibular end-organs are normal at birth in kcne1(-/-) mice, they begin to show modifications during postnatal development: we observed an increase in the height of the dark cells, in their number of mitochondria, and in basolateral membrane infoldings. Subsequently, the epithelium degenerates and the endolymphatic space collapses. Similar changes are known to occur in the cardio-auditory Jervell--Lange-Nielsen syndrome which is caused by mutations in the same channel.

Noninvasive measurement of hydrogen and potassium ion flux from single cells and epithelial structures

This review introduces new developments in a technique for measuring the movement of ions across the plasma membrane. With the use of a self-referencing ion-selective (Seris) probe, transport mechanisms can be studied on a variety of preparations ranging from tissues to single cells. In this paper we illustrate this versatility with examples from the vas deferens and inner ear epithelium to large and small single cells represented by mouse single-cell embryos and rat microglia. Potassium and hydrogen ion fluxes are studied and pharmacological manipulation of the signals are reported. The strengths of the self-referencing technique are reviewed with regard to biological applications, and the expansion of self-referencing probes to include electrochemical and enzyme-based sensors is discussed.

A calcium-activated cation current by an alternatively spliced form of Trp3 in the heart

To investigate a cDNA encoding cation current, we isolated an alternatively spliced form of a rat Trp3, designated Trp3sv. Trp3sv encodes 736 amino acids with a unique N terminus and six transmembrane segments. Expression of the cRNA in Xenopus oocytes was successfully performed. The cation selective current appeared after the addition of ionomycin or induced by prolonged depolarization but not by hyperpolarization. This induction was not observed by a treatment with thapsigargin, phorbol ester, or ATP. Na(+), K(+), tetraethylammonium, and divalent cations were permeable, while N-methylglucamine and chloride were nominally impermeable ions. The currents were not inhibited by flufenamate ruthenium red but nonspecifically by 2 mm Gd(3+). Northern as well as Western blot suggested lower levels of the expression observed in some organs, while reverse transcriptase-polymerase chain reaction suggested that it widely spread among various organs. Therefore, we may conclude that N-terminal spliced valiant of Trp3, Trp3sv, encodes a calcium-activated cation channel in various organs.

K+ and Na+ absorption by outer sulcus epithelial cells

Transduction of sound into nerve impulses by hair cells depends on modulation of a current carried primarily by K+ into the cell across apical transduction channels that are permeable to cations. The cochlear function thus depends on active secretion of K+ accompanied by absorption of Na+ by epithelial cells enclosing the cochlear duct. The para-sensory cells which participate in the absorption of Na+ (down to the uniquely low level of 1 mM) were previously unidentified and the existence of a para-sensory pathway which actively absorbs K+ was previously unknown. A relative short circuit current (Isc,probe, measured as the extracellular current density with a vibrating electrode) was directed into the apical side of the outer sulcus epithelium, decreased by ouabain (1 mM), an inhibitor of Na+, K(+)-ATPase, and found to depend on bath Na+ and K+ but on neither Ca2+ nor Cl-. Isc,probe was shown to be an active current by its sensitivity to ouabain. On-cell patch clamp recordings of the apical membrane of outer sulcus cells displayed a channel activity, which carried inward currents under conditions identical to those used to measure Isc,probe. Both Isc,probe and non-selective cation channels (27.4+/-0.6 ps, n = 22) in excised outside-out patches from the apical membrane were inhibited by Gd3+ (1 mM). Ics,prob was also inhibited by 5 mM lidocaine, 1 mM quinine and 500 microM amiloride but not by 10 microM amiloride. These results demonstrate that outer sulcus epithelial cells contribute to the homeostasis of endolymph by actively absorbing Na+ and K+. An entry pathway in the apical membrane was shown to be through non-selective cation channels that were sensitive to Gd3+.

The m3 muscarinic acetylcholine receptor differentially regulates calcium influx and release through modulation of monovalent cation channels

Several types of transmembrane receptors regulate cellular responses through the activation of phospholipase C-mediated Ca2+ release from intracellular stores. In non-excitable cells, the initial Ca2+ release is typically followed by a prolonged Ca2+ influx phase that is important for the regulation of several Ca2+-sensitive responses. Here we describe an agonist concentration-dependent mechanism by which m3 muscarinic acetylcholine receptors (mAChRs) differentially regulate the magnitude of the release and influx components of a Ca2+ response. In transfected Chinese hamster ovary cells expressing m3 mAChRs, doses of the muscarinic agonist carbachol ranging from 100 nM to 1 mM evoked Ca2+ release responses of increasing magnitude; maximal Ca2+ release was elicited by the highest carbachol concentration. In contrast, Ca2+ influx was maximal when m3 mAChRs were activated by moderate doses (1-10 microM) of carbachol, but substantially reduced at higher agonist concentrations. Manipulation of the membrane potential revealed that the carbachol-induced Ca2+ influx phase was diminished at depolarized potentials. Importantly, carbachol doses above 10 microM were found to couple m3 mAChRs to the activation of an inward, monovalent cation current resulting in depolarization of the cell membrane and a selective decrease in the influx, but not release, component of the Ca2+ response. These studies demonstrate, in one experimental system, a mechanism by which a single subtype of G-protein-coupled receptor can utilize the information encoded in the concentration of an agonist to generate distinct intracellular Ca2+ signals.

Maxi-K+ channel in plasma membrane of basal cells dissociated from the stria vascularis of gerbils

The plasma membrane of isolated strial basal cells has been probed for conductive pathways by the patch-clamp single-channel recording technique. Maxi-K+ channels were identified in 28 excised patches (i.e., 29%) out of 95, and these active patches each contained an average of 2.4 channel activities. In the cell-attached mode, activity of the maxi-K+ channel was also observed. Properties of the maxi-K+ channel thus revealed include: (1) linear I-V relations with 150 mM K+ on both sides of the membrane, (2) a unit conductance of 246.2 +/- 4.0 pS (n = 14). (3) Ca2- sensitivity, (4) activation by membrane depolarization. (5) a complete block by Ba2- (2 mM) from either side of the membrane. (6) a flickering block by quinine (0.1 mM) or verapamil (0.1 mM) from either side of the membrane, and (7) a complete block by tetraethylammonium (1 mM) from the outside only. The maxi-K+ channel may play a role in the generation of endocochlear potentials.

Ion selectivity of volume regulatory mechanisms present during a hypoosmotic challenge in vestibular dark cells

Volume regulation during a hypoosmotic challenge (RVD) in vestibular dark cells from the gerbilline inner ear has previously been shown to depend on the presence of cytosolic K+ and Cl-, suggesting that it involves KCl efflux. The aim of the present study was to characterize hypoosmotically-induced KCl transport under conditions where a hypoosmotic challenge causes KCl influx via the pathways normally used for efflux. Net osmolyte movements were monitored as relative changes in cell volume measured as epithelial cell height (CH). A hypoosmotic challenge (298 to 154 mosM) in the presence of 3.6 or 25 mM K+ and loop-diuretics (piretanide or bumetanide) caused an increase in CH by about a factor of 1.2 presumably due to the net effect of primary swelling defined as osmotic dilution of the cytosol and RVD involving KCl efflux. A hypoosmotic challenge in the presence of 79 mM K+ and loop-diuretics, however, caused CH to increase by a factor of over 2.4. Presumably, this large increase in CH was due to the sum of primary and secondary swelling. Secondary swelling depended on the presence of extracellular K+ and Cl- suggesting that it involved KCl influx followed by water. The ion selectivity of secondary swelling was K+ = Rb+ > Cs+ >> Na+ = NMDG+ and Cl- = NO3- = SCN- >> gluconate-. Secondary swelling was not inhibited by Ba2+, tetraethylammonium, quinidine, lidocaine, amiloride, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, 4-acetamido-4'-diisothiocyanatostilbene-2,2'-disulfonic acid, 4,4'-dinitrostilbene-2,2'-disulfonic acid, 5-nitro-2(3-phenylpropylamino)benzoic acid, acetazolamide, or ethoxyzolamide. These data define a profile of the hypoosmotically-induced KCl transport pathways. The ion selectivity and the blocker insensitivity are consistent with the involvement of the apical slowly activating K+ channel (IsK or minK channel) and the basolateral 360 pS Cl- channel. The involvement of these channels, however, remains to be demonstrated.

Hypo-osmotic challenge stimulates transepithelial K+ secretion and activates apical IsK channel in vestibular dark cells

Volume regulation of vestibular dark cells from the gerbilline inner ear in response to a hypo-osmotic challenge depends on the presence of cytosolic K+ and Cl-. The present study addresses the questions: (i) whether and by what mechanism K+ is released during volume regulation, (ii) whether the osmolarity of the basolateral medium has an effect on the steady-state rate of transepithelial K+ transport and (iii) whether there is cross-talk between the basolateral membrane responsible for K+ uptake and the apical membrane responsible for K+ release. K+ secretion (JK+,probe) and current density (Isc,probe) were measured with vibrating probes in the vicinity of the apical membrane and the transepithelial potential (Vt) and resistance (Rt) were measured in a micro-Ussing chamber. The equivalent short-circuit current (Isc) was calculated. The current (IIsK), conductance (gIsK) and inactivation time constant (tau IsK) of the IsK channel and the apparent reversal potential of the apical membrane (Vr) were obtained with the cell-attached macropatch technique. Vr was corrected (Vrc) for the membrane voltage (Vm) measured separately with microelectrodes. A hypo-osmotic challenge (294 to 154 mosM by removal of 150 mM mannitol) on the basolateral side of the epithelium increased JK+,probe and Isc,probe by a factor of 2.7 and 1.6. When this hypo-osmotic challenge was applied to both sides of the epithelium Vt and Isc increased from 5 to 14 mV and from 189 to 824 microA/cm2 whereas Rt decreased from 27 to 19 omega-cm2. With 3.6 mM K+ in the pipette IIsK was outwardly directed, tau IsK was 267 msec and the hypo-osmotic challenge caused IIsK and gIsK to increase from 14 to 37 pA and from 292 to 732 pS. Vrc hyperpolarized from -44 to -76 mV. With 150 mM K+ in the pipette IIsK was inwardly directed, tau IsK was 208 msec and the hypo-osmotic challenge caused IIsK and gIsK to increase in magnitude from 0 to -21 pA and from 107 to 1101 pS. Vrc remained unchanged (-2 vs. 1 mV). These data demonstrate that a hypo-osmotic challenge stimulates transepithelial K+ secretion and activates the apical IsK channel. The hypo-osmotically-induced increase in K+ secretion exceeded the estimated amount of K+ release necessary for the maintenance of constant cell volume, suggesting that the rate of basolateral K+ uptake was upregulated in the presence of the hypo-osmotic challenge and that cross-talk exists between the apical membrane and the basolateral membrane.

Comparison of ion transport mechanisms between vestibular dark cells and strial marginal cells

Morphologic similarities between strial marginal cells and vestibular dark cells have long been recognized and it has long been accepted that both of these cell types are involved in the secretion of K+ into endolymph. Functional similarities of these two epithelia however, were considered unlikely as long as strial marginal cells were assumed to generate the endocochlear potential which has no equivalent in the vestibular labyrinth. The recently introduced concept that strial marginal cells transport K+ but that the mechanism for the generation of the endocochlear potential is located in another cell type provided the basis to hypothesize that ion transport mechanisms and their regulation are similar in vestibular dark and strial marginal cells. The present review compiles evidence in support of this hypothesis.

Amiloride-sensitive Na+ channel-like immunoreactivity in the luminal membrane of some non-sensory epithelia of the inner ear

Some non-sensory epithelia of the inner ear were examined for the localization of immunoreactivity to polyclonal antibodies raised against amiloride-sensitive Na+ channels from the bovine kidney. The pre-embedding immunogold technique was used for this purpose. Labelings were found on the membrane of the endolymphatic surface of strial marginal cells, epithelial cells of spiral prominence and Reissner's membrane, and ampullar dark cells. In contrast, no labeling was found on the luminal membrane of mesothelial cells of Reissner's membrane, the cells lining the supra-strial perilymphatic space, transitional cells and ampullar ceiling cells. Since the antibodies used may also label non-selective cation channels and non-functional sodium channel precursors as suggested by others, it was not possible to determine the labelings are solely due to amiloride-sensitive Na+ channels. However, the observed result was consistent with the previous studies that amiloride blocks ion transport in strial marginal cells and the semicircular canal. It is therefore likely that the observed labeling includes amiloride-sensitive Na+ channels. These labeled ion channels in a variety of epithelial cells lining the endolymphatic space could be important in the inner ear fluid regulation.

Ion transport mechanisms responsible for K+ secretion and the transepithelial voltage across marginal cells of stria vascularis in vitro

It has long been accepted that marginal cells of stria vascularis are involved in the generation of the endocochlear potential and the secretion of K+. The present study was designed to provide evidence for this hypothesis and for a cell model proposed to explain K+ secretion and the generation of the endocochlear potential. Stria vascularis from the cochlea of the gerbil was isolated and mounted into a micro-Ussing chamber such that the apical and basolateral membrane of marginal cells could be perfused independently. In this preparation, the transepithelial voltage (Vt) and resistance (Rt) were measured across marginal cells and the resulting equivalent short circuit current (Isc) was calculated (Isc = Vt/Rt). Further, K+ secretion (JK+,probe) was measured with a K(+)-selective vibrating probe in the vicinity of the apical membrane. In the absence of extrinsic chemical driving forces, when both sides of the marginal cell epithelium were bathed with a perilymph-like solution, Vt was 8 mV (apical side positive), Rt was 10 ohm-cm2 and Isc was 850 microA/cm2 (N = 27). JK+,probe was outwardly directed from the apical membrane and reversibly inhibited by basolateral bumetanide, a blocker of the Na+/Cl-/K+ cotransporter. On the basolateral but not apical side, oubain and bumetanide each caused a decline of Vt and an increase of Rt suggesting the presence of the Na,K-ATPase and the Na+/Cl-/K+ cotransporter in the basolateral membrane. The responses to [Cl-] steps demonstrated a significant Cl- conductance in the basolateral membrane and a small Cl- conductance in the paracellular pathway or the apical membrane. The responses to [Na+] steps demonstrated no significant Na+ conductance in the basolateral membrane and a small Na+ or nonselective cation conductance in the apical membrane or paracellular pathway. The responses to [K+] steps demonstrated a large K+ conductance in the apical membrane. Apical application of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and basolateral elevation of K+ caused an increase in Vt and a decrease in Rt consistent with stimulation of the apical K+ conductance. Similar observations have been made in vestibular dark cells, which suggest that strial marginal cells and vestibular dark cells are homologous and transport ions by the same pathways. Taken together, these observations are incompatible with a model for the generation of the endocochlear potential which ascribes the entire potential to the strial marginal cells [Offner et al. (1987) Hear. Res. 29, 117-124].(ABSTRACT TRUNCATED AT 400 WORDS)

Ion channels in basolateral membrane of marginal cells dissociated from gerbil stria vascularis

The basolateral membrane of isolated strial marginal cells has been probed for conductive pathways by the patch-clamp technique. Two types of voltage-insensitive channels were identified in both cell-attached and excised patches. Of these, frequently (69% of excised patches) observed was a Ca(2+)-activated nonselective cation channel having a unit conductance of 24.9 +/- 0.5 pS (N = 16). Other characteristics of this type in excised patches include: 1) linear I-V relations with 150 mM K+ (pipette)/150 mM Na+ (bath), 2) a permeability sequence of NH4+ > Na+ = K+ = Rb+ > Li+, 3) a flickering block by quinine or quinidine (both 1 mM), and 4) a dose dependent block of its activity by ADP or ATP (IC50,ATP/IC50,ADP = 20-35), both from the cytosolic side. Channels with similar characteristics were found in the apical membrane of the same cell; however, the basolateral channels were 2-4 times more densely distributed than the apical counterparts. Also frequently (57%) detected was a Cl- channel of 80.0 +/- 0.5 pS (N = 6), whose activity was Ca2+ independent. Additionally, this Cl- channel had: 1) linear I-V relations with symmetric Cl-, 2) a permeability sequence of Cl- > Br- > I- > or = NO3- > or = gluconate-, and 3) a complete and reversible block by 1 mM diphenylamine-2-carboxylate. In contrast to the apical Cl- channels, the basolateral ones had a much higher density (57% vs. < 1%) as well as a higher unit conductance (80 pS vs. 50 pS) than the apical counterpart. The relative abundance of these two types as the major conductive pathways for Na+, K+, and Cl- in the basolateral region must be taken into account when addressing the role of strial marginal cells in generating the positive endocochlear potential. The Cl- channel may facilitate Cl- distribution across the basolateral membrane.

Monovalent cation selective channel in the apical membrane of rat inner medullary collecting duct cells in primary culture

Ion channels in the apical membrane of rat inner medullary collecting duct (IMCD) were investigated by the patch clamp technique. Owing to the histological heterogeneity of IMCD, cells were cultured from the lower half of the inner medulla of Wistar rat kidney. Channel activity was rarely seen in cell attached patch, but membrane excision activated multiple units of 28.2 +/- 0.7 pS cation selective channel. A Na or K selective channel was not found. The 28 pS channel showed membrane voltage dependency, no rectification, almost equal permeability to monovalent cations (Na/K/Li/Cs/Rb/NH4 = 1:1.00:0.82:0.97:1.10:1.71) and no significant permeation to anions or divalent cations. Calcium of the cytoplasmic side from 10(-7) M to 10(-4) M affected the mean number of open channels (nPo) dose-dependently in excised patch (IC50 = 5 x 10(-6) M). 1 mM of ATP, ADP, AMP and gadolinium reversibly suppressed nPo to near zero whereas amiloride, cAMP or cGMP had no effect. Multiple conductance substates were frequently observed. These results suggested that this channel belongs to the nonselective cation channels which has been identified in other epithelia and is not responsible for amiloride sensitive Na transport through IMCD cells.

Potassium secretion by vestibular dark cell epithelium demonstrated by vibrating probe

Detection of motion and position by the vestibular labyrinth depends on the accumulation of potassium within a central compartment of the inner ear as a source of energy to drive the transduction process. Much circumstantial evidence points to the vestibular dark cell (VDC) epithelium as being responsible for concentrating K+ within the lumen. We have used the vibrating probe technique to directly observe voltage and ion gradients produced by this tissue to put this assumption on a solid experimental footing. Relative current density (Isc,probe) over the apical membrane of VDC epithelium was measured with the vibrating voltage-sensitive probe, and this technique was validated by performing maneuvers known to either stimulate or inhibit the transepithelial equivalent short circuit current. Basolateral bumetanide (5 x 10(-5) M) and ouabain (1 x 10(-3) M) caused a decrease in Isc,probe by 55 +/- 6% and 39 +/- 3%, respectively while raising the basolateral K+ concentration from 4 to 25 mM caused an increase by 35 +/- 8%. A K+ gradient directed toward the apical membrane was detected with the vibrating K(+)-selective electrode, demonstrating that, indeed, the VDC epithelium secretes K+ under control conditions. This secretion was inhibited by bumetanide (by 94 +/- 7%) and ouabain (by 52 +/- 8%). The results substantiate the supposition that dark cells produce a K+ flux and qualitatively support the correlation between this flux and the transepithelial current.

A calcium-activated nonselective cationic channel in the basolateral membrane of outer hair cells of the guinea-pig cochlea

The patch-clamp technique was used to investigate ion channels in the basolateral perilymph-facing membrane of freshly isolated outer hair cells (OHCs) from the guinea-pig cochlea. These sensory cells probably determine, via their motile activity, the fine tuning of sound frequencies and the high sensitivity of the inner ear. A Ca(2+)-activated nonselective cationic channel was found in excised inside-out membrane patches. The current/voltage relationship was linear with a unit conductance of 26.3 +/- 0.3 pS (n = 15) under symmetrical inger conditions. The channel excluded anions (PNa/PCl = 18 where PNa/PCl denotes the relative permeability of Na to Cl); it was equally permeant to the Na+ and K+ ions and exhibited a low permeability to N-methyl-D-glucamine and Ba2+ or Ca2+. Channel opening required a free Ca2+ concentration of about 10(-6) mol/l on the internal side of the membrane and the open probability (Po) was maximal at 10(-3) mol/l (Po = 0.72 +/- 0.06, n = 12). Adenosine 5'mono-, tri- and di-phosphate reduced Po to 29 +/- 14 (n = 5), 42 +/- 10 (n = 8) and 51 +/- 12 (n = 5) % of control Po, respectively, when they were added at a concentration of 10(-3) mol/l to the internal side. The channel was partially blocked by flufenamic acid (10(-4) mol/l) and 3',5'-dichlorodiphenylamine-2-carboxylic acid (DCDPC, 10(-5) mol/l). This type of channel, together with Ca(2+)-activated K+ channels, might participate in the control of membrane potential and modulate the motility of OHCs.

Transepithelial voltage and resistance of vestibular dark cell epithelium from the gerbil ampulla

Transepithelial voltage (Vt) and resistance (Rt) were measured across the dark cell epithelium of the gerbil ampulla using a micro Ussing chamber of improved design in order to test the view that the histologically similar epithelia in the utricle and in the ampullae exhibit similar electrophysiologic functions. Vt was found to be 8.0 +/- 0.3 mV and Rt was 11.6 +/- 0.4 ohm-cm2 (N = 179) when both sides of the tissue were perfused with symmetric perilymph-like solution. The equivalent short circuit current (Isc = Vt/Rt) was 712 +/- 18 microA/cm2 (N = 179). Isc was reduced from 638 +/- 60 to 48 +/- 16 microA/cm2 (N = 14) by basolateral perfusion of 10(-3) M ouabain and from 538 +/- 27 to 27 +/- 4 microA/cm2 (N = 15) by basolateral perfusion of 5 x 10(-5) M bumetanide. Basolateral K+ steps (Na+ substitution) from 3.6 to 25 mM increased Vt from 6.5 +/- 0.5 to 12.2 +/- 0.6 mV and reduced Rt from 9.7 +/- 0.7 to 7.4 +/- 0.5 ohm-cm2 (N = 43). Apical K+ steps from 3.6 to 25, to 100 mM or to 145 mM led to a decrease in both Vt and Rt. The steady state Vt during apical perfusion of 145 mM K+ was near zero. Upon return to 3.6 mM K+, Vt transiently overshot its original level. Apical Cl- steps from 150 to 50 mM (gluconate substitution) monophasically decreased Vt from 5.9 +/- 0.7 to 4.1 +/- 0.8 mV (N = 15) and increased Rt from 9.6 +/- 1.3 to 12.0 +/- 1.5 ohm-cm2 (N = 14).(ABSTRACT TRUNCATED AT 250 WORDS)

Ba2+ and amiloride uncover or induce a pH-sensitive and a Na+ or non-selective cation conductance in transitional cells of the inner ear

The membrane potential Vm the cytosolic pH (pHi), the transference numbers (t) for K+, Cl- and Na+/non-selective cation (NSC) and the pH-sensitivity of Vm were investigated in transitional cells from the vestibular labyrinth of the gerbil. Vm, pHi, tK+, tCl-, tNa+/NSC, and the pHi sensitivity of Vm were under control conditions were -92 +/- 1 mV (n = 89 cells), pHi 7.13 +/- 0.07 (n = 11 epithelia), 0.87 +/- 0.02 (n = 22), 0.02 +/- 0.01 (n = 19), 0.01 +/- 0.01 (n = 24) and -5 mV/pH unit (n = 13 cells/n = 11 epithelia), respectively. In the presence of 100 mumol/l Ba2+ the corresponding values were: -70 +/- 1 mV (n = 32), pHi 7.16 +/- 0.08 (n = 6), 0.31 +/- 0.05 (n = 4), 0.06 +/- 0.01 (n = 6), 0.20 +/- 0.03 (n = 10) and -16 mV/pH-unit (n = 15/n = 6). In the presence of 500 mumol/l amiloride the corresponding values were: -72 +/- 2 mV (n = 34), pHi 7.00 +/- 0.07 (n = 5), 0.50 +/- 0.04 (n = 6), 0.04 +/- 0.01 (n = 11), 0.28 +/- 0.04 (n = 9) and -26 mV/pH-unit (n = 20/n = 5). In the presence of 20 mmol/l propionate plus amiloride the corresponding values were: -61 +/- 2 mV (n = 27), pHi 6.72 +/- 0.06 (n = 5), 0.30 +/- 0.02 (n = 6), 0.06 +/- 0.01 (n = 5) and 0.40 +/- 0.02 (n = 8), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of mesangial cell ion channels by insulin and angiotensin II. Possible role in diabetic glomerular hyperfiltration

We used patch clamp methodology to investigate how glomerular mesangial cells (GMC) depolarize, thus stimulating voltage-dependent Ca2+ channels and GMC contraction. In rat GMC cultures grown in 100 mU/ml insulin, 12% of cell-attached patches contained a Ca(2+)-dependent, 4-picosiemens Cl- channel. Basal NPo (number of channels times open probability) was < 0.1 at resting membrane potential. Acute application of 1-100 nM angiotensin II (AII) or 0.25 microM thapsigargin (to release [Ca2+]i stores) increased NPo. In GMC grown without insulin, Cl- channels were rare (4%) and unresponsive to AII or thapsigargin in cell-attached patches, and less sensitive to [Ca2+]i in excised patches. GMC also contained 27-pS nonselective cation channels (NSCC) stimulated by AII, thapsigargin, or [Ca2+]i, but again only when insulin was present. In GMC grown without insulin, 15 min of insulin exposure increased NPo (insulin > or = 100 microU/ml) and restored AII and [Ca2+]i responsiveness (insulin > or = 1 microU/ml) to both Cl- and NSCC. GMC AII receptor binding studies showed a Bmax (binding sites) of 2.44 +/- 0.58 fmol/mg protein and a Kd (binding dissociation constant) of 3.02 +/- 2.01 nM in the absence of insulin. Bmax increased by 86% and Kd was unchanged after chronic (days) insulin exposure. In contrast, neither Kd nor Bmax was significantly affected by acute (15-min) exposure. Therefore, we concluded that: (a) rat GMC cultures contain Ca(2+)-dependent Cl- and NSCC, both stimulated by AII. (b) Cl- efflux and cation influx, respectively, would promote GMC depolarization, leading to voltage-dependent Ca2+ channel activation and GMC contraction. (c) Responsiveness of Cl- and NSCC to AII is dependent on insulin exposure; AII receptor density increases with chronic, but not acute insulin, and channel sensitivity to [Ca2+]i increases with both acute and chronic insulin. (d) Decreased GMC contractility may contribute to the glomerular hyperfiltration seen in insulinopenic or insulin-resistant diabetic patients.

Activation and modulation of calcium-activated non-selective cation channels from embryonic chick sensory neurons

We have shown that calcium-activated non-selective (CAN) channels from embryonic chick sensory neurons are permeable to both Na+ and K+ and are not blocked by TTX, TEA, or 4-AP. These neuronal CAN channels are activated by sub-micromolar cytoplasmic Ca2+ with negative cooperativity. The effect of Ca2+ is to decrease the closed times of the channel with little effect on the time the channel remains open. Isolated neuronal CAN channels can be phosphorylated by cAMP-dependent protein kinase (PKA). The effect of phosphorylation is to shorten channel open time and to minimize the effect of Ca2+ on channel closed time.

Two types of chloride channel in the basolateral membrane of vestibular dark cells

Transepithelial and cell membrane potential measurements have suggested that the basolateral membrane of gerbil vestibular dark cells contains Cl- conductive pathways. We used the patch clamp technique to search this membrane for Cl- conductive channels which could account for the macroscopic observations. Two types of Cl- channel were found in both cell-attached and excised membrane patches. One type was found with an incidence of 19% and had a single-channel conductance of 95 +/- 1 pS (N = 20) in symmetrical Cl- solutions. The other type was found with an incidence of 3% and had a large single-channel conductance of 360 +/- 11 pS (N = 12) in symmetrical Cl- solutions (LC-type Cl- channel). Both types of Cl- channel had linear current-voltage relations and at least 2 substates. In asymmetrical Cl- solutions (gluconate substitution) the current-voltage relations fit the Goldman-Hodgkin-Katz current equation for Cl-. Neither channel was blocked by Zn2+, NPPB, DIDS, DNDS or quinine. The 95 pS channel exhibited a spontaneous 'rundown' of its activity within 1 to 10 min after being excised. This rundown was not reversed by the catalytic subunit of protein kinase A. Channel activity was not dependent on the presence of cytosolic Ca2+ nor markedly altered by variations in cytosolic pH between 6.5 and 8.0. The two Cl- channels were distinguished by the membrane voltage ranges in which they were active and by their anion selectivity. The open probability of the 95 pS channel was insensitive to voltage and the anions NO3-, I- and Br- were only half as permeable as Cl-. By contrast, the LC-type Cl- channel was mostly active between about +/- 30 mV and equally permeable to NO3-, I-, Br- and Cl-. The 95 pS Cl- channel may account for the observed transepithelial and intracellular voltage responses to Cl- concentration steps and provide the path for the recirculation of Cl- across the basolateral membrane. The LC-type Cl- channel shows the same lack of anion discrimination as the anion pathway activated during hyposmotic challenge.

Aminoglycoside antibiotics inhibit maxi-K+ channel in single isolated cochlear efferent nerve terminals

Patch clamp recordings were obtained from isolated cochlear efferent nerve terminals. The effect of aminoglycoside antibiotics on single maxi-K+ channels was determined. At positive voltages (cytosol with respect to extracellular side), neomycin, streptomycin, and kanamycin significantly reduced the single channel current amplitude of the maxi-K+ channel from the cytosolic side. The IC50 for neomycin was 9.10(-4) M from the cytosolic side and >> 10(-3) M from the extracellular side. Streptomycin and kanamycin were less potent. No significant difference in inhibition of the single channel current amplitude by 2.5.10(-4) M cytosolic neomycin was observed between 7.10(-4) M and 10(-6) M free cytosolic Ca2+. Neomycin had no significant effect on the open probability of the maxi-K+ channel either from the cytosolic or from the extracellular side. These findings demonstrate that the maxi-K+ channel in cochlear efferent nerve terminals can be a site of action for aminoglycoside antibiotics.

Maxi-K+ channel in single isolated cochlear efferent nerve terminals

Patch clamp recordings were obtained from isolated cochlear efferent nerve terminals. Channel activity was found in 85% of membrane patches, was present in on-terminal and excised patches and was characterized to originate from a maxi-K+ channel. An average of 2.0 +/- 0.1 (N = 33) maxi-K+ channels were found per active patch. In symmetrical solutions, the current-voltage relationship was linear and the single-channel conductance was 221 +/- 5 pS (N = 22). The open probability of the maxi-K+ channel increased with depolarization of the membrane potential and with an increasing free Ca2+ concentration on the cytosolic side. The open probability was insensitive to changes in the free Ca2+ concentration on the extracellular side. TEA (20 mM) and charybdotoxin (10(-7) M) decreased the open probability to nearly zero from the extracellular side but had no effect from the cytosolic side. The high incidence with which this channel was found suggests that the maxi-K+ channel is physiologically relevant which might include protection against overstimulation of the efferent synapse.

Poorly selective cation channels in apical membranes of epithelia

The apical membrane of frog skin contains two types of pathways which allow the passage of several monovalent cations in the absence of external Ca2+. Differences between the two pathways concern their open-close kinetics, selectivity, and the affinity for several blocking agents. Type S channels open and close relatively slowly, whereas type F channels display fast open-close kinetics. Both channel types allow the passage of Na+, K+, and Rb+ currents which are blocked by divalent cations and La3+ added to the extracellular side. Type F channels are permeable for Cs+ which is, however, excluded from type S channels. Shifts in open-close kinetics induced by Mg2+ occur at concentrations below 5 microM for type F channels, whereas more than a tenfold higher dose is required for the type S pathway. UO2(2+) concentrations up to 100 microM only occlude type S channels while 100 microM tetracaine selectively blocks type F channels. Apical membranes of toad urinary bladder, cultured amphibian renal epithelia (A6), and toad colon contain only type F channels. In toad bladder and A6 cells volume expansion strongly activates this pathway. Macroscopic currents carried by Ba2+ and Ca2+ could be recorded after activation of toad bladders with oxytocin and treatment of the apical surface with nanomolar concentrations of Ag+, which seems to interact with a site located at the channel interior.

The membrane potential of vestibular dark cells is controlled by a large Cl- conductance

The K+ secretory epithelium of the vestibular labyrinth (dark cells) was impaled with glass microelectrodes in order to test the hypothesis that it contains a large Cl- conductance. In the first series of experiments, the short-circuited epithelium was perfused on both sides by a solution containing 150 mmol/l Cl-. The membrane voltage (PD) was -18 +/- 1 mV (N = 101), showed a Gaussian distribution, and the estimated input resistance of the cell (R 'cell') was 17 +/- 3 M omega. The PD responded to 10(-4) mol/l ouabain with a depolarization, suggesting the presence of a (Na(+) + K+)-ATPase. The PD responses to Cl- steps yielded an apparent transference number tCl = 0.34 +/- 0.03 (N = 65) and those to K+ steps yielded a tK = 0.16 +/- 0.01 (N = 48). In the second series of experiments, cells presumed to be Cl(-)-depleted were impaled in Cl(-)-free solutions. The distribution of the PD was not Gaussian; PDs as negative as -90 mV were observed. Cells with a highly negative PD also had a high R 'cell'. With the addition of Cl- the PD collapsed to -19 +/- 1 mV and R collapsed to 16 +/- 3 M omega (N = 145) which are not significantly different from values obtained in the first series of experiments when cells were impaled in a solution containing 150 mmol/l Cl-. Alternating the bath perfusate between Cl(-)-free and Cl(-)-containing solutions led to large PD transients.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca(2+)-activated nonselective cation, maxi K+ and Cl- channels in apical membrane of marginal cells of stria vascularis

Patch clamp recordings on the apical membrane of marginal cells of the stria vascularis of the gerbil were made in the cell-attached and excised configuration. Marginal cells are thought to secrete K+ into and absorb Na+ from endolymph. Four types of channel were identified; the most frequently observed channel was a small, nonselective cation channel which was highly similar to that found in the apical membrane of vestibular dark cells (Marcus et al., (1992) Am. J. Physiol. 262, C1423-C1429). The small nonselective cation channel was equally conductive (26.7 +/- 0.3 pS; N = 49) for K+, Na+, Rb+, Li+ and Cs+, 1.6 times more permeable to NH4+, but not permeable to Cl-, Ca2+, Ba2+ or N-methyl-D-glucamine. This channel yielded linear current-voltage relations which passed nearly through the origin (intercept: -2.2 +/- 0.4 mV, N = 49) when conductive monovalent cations were present on both sides of the membrane in equal concentrations. Channel activity required the presence of Ca2+ at the cytosolic face but not the extracellular (endolymphatic) face; there was essentially no activity for cytosolic Ca2+ less than or equal to 10(-7) M Ca2+ and full activity for greater than or equal to 10(-5) M. Cell-attached recordings had a conductance of 28.6 +/- 2.2 pS (N = 6) and a reversal voltage of -2.2 +/- 5.2 mV (N = 3) which was interpreted to reflect the intracellular potential of marginal cells under the present conditions. The three other types of channel were a Cl- channel (approximately 50 pS; N = 2), a maxi-K+ channel (approximately 230 pS; N = 1), and another large channel, probably cation nonselective (approximately 170 pS; N = 1). The 27 pS nonselective cation channel may be involved in K+ secretion and Na+ absorption under stimulated conditions which produce an elevated intracellular Ca2+; however, consideration of the apparent channel density in relation to the total transepithelial K+ flux suggests that these channels are not sufficient to account for K+ secretion.